1. Technical Field
This invention is related to fiber optic sensors, and more particularly to hydrophones and dynamic pressure sensors for use in an underwater environment.
2. Background Technology
Most commercially available hydrophones typically use a piezoelectric crystal as the sensing element. When pressure is applied on the PZT, a small surface electrical charge is generated, resulting in a small electrical voltage. The small electrical signal is typically amplified at the sensor. The electrical signal and the electronic amplifier make the device sensitive to electromagnetic interference. In addition, such PZT sensors are generally not very sensitive at frequencies below about 1 Hz.
Fiber optic sensors have also been used for sensing pressure, temperature, strain, displacement, acceleration, bending, and other environmental properties.
A fiber optic hydrophone based on a birefringent material is discussed in W. B. Spillman and D. H. McMahon, “Multimode fiber-optic hydrophone based on the photoelastic effect”, Applied Optics, Vol. 21, No. 19, pp. 33511-3514, (October 1982), and in D. H. McMahon, R. A. Soref, and L. E. Sheppard, “Sensitive Fieldable Photoelastic Fiber-Optic Hydrophone”, Journal of Lightwave Technology, Vol. LT-2, No. 4, pp. 469-478, (August 1984). Another fiber optic hydrophone is disclosed in W. B. Spillman and D. H. McMahon, “Frustrated-total-internal-reflection multimode fiber-optic hydrophone” Applied Optics, Vol. 19, No. 1, pp. 113-117, (1980). Another hydrophone is discussed in W. B. Spillman, Jr., “Multimode fiber-optic hydrophone based on a schlieren technique”, Applied Optics, Vol. 20, No. 3, pp. 465-470, (1981).
Interferometric fiber-optic sensors are also disclosed in U.S. Pat. No. 5,625,724 to Frederick et al., which uses both a reference fiber and a sensing fiber wrapped around a rigid cylinder, and a sensing fiber wrapped around a compliant material. Sensitivity of fiber optic hydrophones is discussed in P. Shajenko, J. P. Flatley, M. B. Moffett, “On fiber-optic hydrophone sensitivity” Journal of the Acoustic Society of America, Vol. 64, No. 5, pp. 1286-1288, November 1978. A more recent interferometric fiber optic hydrophone is disclosed in Z. Wang and Y. Hu, “Frequency response of fiber-optic hydrophone with a novel mechanical anti-aliasing filter of side-cavities”, Communications and Photonics Conference and Exhibition, 2009 Asia, Proceedings of SPIE, Vol. 7630, pp. 763024-1-763024-5, November 2009. Another fiber optic hydrophone is described in U.S. Pat. No. 7,466,631 to Ames, entitled “Enhanced Sensitivity Pressure Tolerant Fiber Optic Hydrophone”.
Various types of intensity modulated fiber optic sensors are disclosed in U.S. Pat. No. 6,998,599 to Lagakos et al., U.S. Pat. No. 7,379,630 to Lagakos et al., U.S. Pat. No. 7,460,740 to Lagakos et al., U.S. Pat. No. 7,020,354 to Lagakos et al., U.S. Pat. No. 7,697,798 to Lagakos et al., and U.S. Patent Application Publication 20090196543, the disclosures of which are incorporated herein by reference in their entireties.
A multimode fiber optic acoustic sensor is described in M. R. Layton and J. A. Bucaro, “Optical fiber acoustic sensor utilizing mode-mode interference”, Applied Optics, Vol. 18, No. 5, pp. 666-670, (March 1979).
A microbend sensor suitable is described in N. Lagakos, J. H. Cole, and J. A. Bucaro, “Microbend fiber-optic sensor”, Applied Optics, Vol. 26, No. 11, pp. 2171-2180, (June 1987). Other fiber optic sensors are described in Bucaro J. A., et al., “Fiber Optic Hydrophone”, Journal of Acoustical Society of America, Vol. 62, pp. 1302-1304, 1977; Cole, J. H., et al., “Fiber Optic Detection of Sound”, Journal of Acoustic Society of America, Vol. 62, pp. 1136-1138, 1977; and T. G. Giallorenzi, J. A. Bucaro, A. Dandridge, G. H. Sigel, J. H. Cole, S. C. Rashleigh and R. G. Priest, “Optical fiber sensor technology,” IEEE Transactions on Microwave Theory and Techniques MTT-30, pp. 472-511, (1982).